The present invention relates to a propulsion unit or thruster particularly for controlling and maneuvering spacecraft.
British Pat. No. 1,473,749 describes a thruster of the type to which the invention pertains which includes a chamber for decomposing a propellent fuel such as hydrazine. A heating device is disposed downstream from the decomposition chamber proper for additionally heating and additinally decomposing the propellent gases produced in the decomposition chamber. A gas ejection, jet producing nozzle is connected to that heater for accelerating and ejecting the propellant gases for producing thrust.
Thrusters of the type described are used particularly for attitude control, orbit correction, station keeping, repositioning and other space craft maneuvers. If hydrazine is used, (N.sub.2 H.sub.4) this fuel can be decomposed either by a catalytic reaction or as specifically disclosed in the above identified British patent, the decomposition reaction may be thermally induced causing the hydrazine to decompose into the gaseous decomposition products NH.sub.3, N.sub.2, and H.sub.2. The decompositioning reaction is an exothermic one developing in fact a considerable amount of heat i.e. useful enthalpy. The gases as produced will in turn produce the thrust necessary for the type of maneuvers mentioned above.
The British Pat. No. 1,473,749 discloses further that the thrust can be enhanced if the gasseous reaction and decomposition products are heated further and if the ammonia gas NH.sub.3 resulting from the primary decomposition process is further decomposed into nitrogen and hydrogen. This particular reaction is an endothermic one. The gases, therefore, are ejected only after this secondary reaction. In fact, the gases so ejected have a still higher speed which is a result of a pressure increase. This pressure increase, in turn, is a result of the supplemental heating in fact increasing the kinetic energy of the propulsion gases; on the other hand, the speed is increased as a result of the changes in the composition of the gas mixture as a result of the ammonia decomposition as a secondary reaction. As stated above, the British patent suggests a supplemental heating chamber for purposes of providing the additional heating and secondary decompositioning which supplemental chamber contains and supports a spirally coiled metal tube. The propulsion and reaction gases flow through that tube and are heated therein by means of electrical resistance heating. The British patent discloses further an alternative construction in which the propulsion gases flow tangentially into a cylindrical heating chamber containing in its center a heating coil.